1. Field of the Invention
The present invention relates to a lens, a camera device, and a handheld terminal.
The lens is suitably used as a lens for photo shooting by a digital camera, a digital video camera, or a silver-halide film camera.
Therefore, the camera of the present invention may be a digital camera, a digital video camera, or a silver-halide film camera.
In addition, a handheld terminal using the digital camera and/or the digital video camera as the imaging function that processes information such as communication is provided.
2. Discussion of the Background
As digital cameras have become widely diffused, the performance and capabilities thereof have been improved and various types of cameras are now available in the market.
Among these, a number of users demand a quality compact-size camera having a high-performance single focus lens, and are waiting for the emergence of a camera product with a lens of large diameter having a small F number in addition to high performance.
Here, high performance means, for example, a resolution corresponding to an image pick-up of from at least 10,000,000 to 20,000,000 pixels, less coma flare when the aperture stop is fully open, less point image collapse to the periphery of the field angle of view under high contrast, less chromatic aberration resulting in no unneeded coloring in a portion having a large luminance difference, and less distortion aberration under which a straight line can be depicted as a straight line.
In addition, with regard to the large-diameter lens, an f number of at least F2.4 or less and preferably F2.0 or less is greatly requested to distinguish it from a compact camera on which a zoom lens is mounted.
Furthermore, with the angle of field, a half angle of field of 38 degrees or more corresponding to a focal length of 28 mm in conversion of a 35 mm silver salt film camera (“Leica version”) is preferable.
As a specific lens arrangement of a wide-angle single focus lens there is a “retrofocus” type, in which a lens group (front element) having a negative refractive power is located on the object side and a lens group (rear element) having a positive refractive power is located on the image side.
The retrofocus type is suitably used as an image focus lens arrangement for a digital camera, a digital video camera, etc., using an area sensor having a color filter or a micro lens for each pixel, because the exit pupil can be positioned away from the image to make the peripheral beams of light enter into the image surface at an angle close to a right angle.
However, the retrofocus type has a large refractive power arrangement asymmetry, which tends to cause incomplete correction of coma aberration, distortion aberration, chromatic aberration of magnification, etc.
Focus lenses of the retrofocus type are of great variety. Unexamined published Japanese patent application Nos. (hereinafter referred to as JP) H06-308385-A, 2006-349920-A and H09-218350-A describe lenses having a half angle of field of around 38 degree with a large diameter.
Although the image focusing lenses described in JP H06-308835-A have a large diameter (F1.4), astigmatism and field curvature of the lenses are also large, meaning that performance at the periphery when the aperture stop is fully open tends to be problematic. Thus, the lens is thought to be not suitable for an image pick-up element of 10,000,000 to 20,000,000 pixels.
The image focusing lenses described in JP 2006-349920-A are suitably corrected with regard to astigmatism, field curvature and distortion aberration but with a slightly large spherical aberration.
Demand for greater compactness of digital cameras and digital video cameras remains stronger than ever. Thus, although generally successful, the lens having a small f number as described in JP 2006-349920-A is slightly insufficient considering the level of compactness now required.
The lenses described in JPs H09-218350-A and H07-46337-A has F2.8, meaning that the lenses do not satisfy recent demand. In addition, correction of astigmatism, field curvature, and chromatic aberration of magnification is insufficient in terms of the lens performance out to the peripheral.
Any of the lenses described in JPs H06-308385-A and H09-218350-A has a distortion aberration greater than 2% in absolute value.
JP 2008-129403-A describes an image forming lens having F2.9, meaning that the lens is insufficient in terms of the diameter thereof. In addition, the field curvature of the lens is too large to secure good performance out to the periphery. Furthermore, a great many lenses are required and thus size reduction is not easy to achieve.
In addition, although digital cameras have been reduced in size in recent years even further reductions in size have been demanded.
Accordingly, with regard to the compact size digital camera, a storing (retraction) mechanism is proposed in which the entire photographic optical system is retracted into the camera body by reducing the space between the lens groups and lenses of the photographic optical system so as to avoid protrusion of the optical system from the camera body when the camera is not in use.
However, an increase of the diameter of the lens leads to an increase in the number of the lenses in the system, resulting in an increase in the total thickness of the lenses. Thus, the length of the lens barrel increases irrespective of any reduction in the space between the lens groups and/or the lenses on the optical axis by the storing mechanism, with the result that the lens barrel may not be sufficiently contained in the camera body.
JP 2006-39152-A describes a technology of retracting a photographic optical system into the body of a digital camera, a digital video camera, or a handheld terminal, in which the lenses constituting the system is retracted from the optical axis. However, JP 2006-39152-A makes no specific mention of the photographic system to be retracted into the body of a camera, etc. Therefore, further refinements are required to deal with a case in which the number of lenses in the photographic optical system increases as a result of an increase in the diameter of the lens.